A follow-up clinical examination is proposed in approximately 700 female twins who participated in a previous twin study of coronary heart disease (CHD) risk factors at Kaiser Permanente. During the ten years since this first examination, several new or more specific CHD risk factors has been recognized, including low density lipoprotein subclass patterns, plasma apolipoprotein and insulin levels, and the anatomic distribution of body fat. The proposed study examine evidence of genetic and environmental or behavioral influences on these newly identified risk factors. Plasma lipoprotein levels, LDL and HDL subclasses, and apolipoproteins AI and B will be measured at this examination. Fasting and postload serum insulin and glucose will be determined and several anthropometric measures of body fat distribution obtained. A submaximal treadmill exercise test will be performed, if not medically contraindicated, to assess cardiovascular fitness and three questionnaires will be administered to record physical activity level, dietary intake, and medical history. Information will be obtained from both returnees and nonreturnees. To assess genetic influence, the heritability of each of the risk factors will be estimated. Blood will also be frozen for future studies of polymorphic DNA markers. Using the monozygotic co-twin control method, the effects of environmental or behavioral factors on these CHD risk factors will be assessed after controlling for genetic factors that could explain their associations. Relationships between CHD CHD risk factors will be assessed with similar methods. We will also examine the hypotheses that can be uniquely addressed using twin data: first, that the association of body fat distribution with other CHD risk factors is mediated by serum insulin levels; second, that cardiovascular fitness explains, at least in part, the association of physical activity with CHD risk factors such as HDL-cholesterol and insulin levels. Because most previous studies of this nature have had men as subjects, this study of women should provide important information for understanding gender-related differences in these risk factors and their associations.